This chapter explains in the beginning as to how light played a pivotal role not only in the creation of life but also in the evolution of human thoughts over centuries that shaped our understanding of nature. Elemental knowledge of the nature and properties of light is an essential prerequisite for understanding a laser. The content of this chapter has been specifically planned not only to impart basic knowledge of light and its behavioral pattern and properties but also to kindle an enthusiasm in the mind of the readers. It is a common knowledge that light travels in a straight path, or else there would not be occurrence of day and night or lunar and solar eclipses, or formation of shadows, for that matter. The rectilinear motion of light has manifested in a variety of phenomena such as reflection, refraction, dispersion, interference, and scattering to name but a few. There is no denying of the fact that light and its various actions are profoundly interlaced with our day-to-day life. Consequently, real-life examples, terrestrial or celestial, are often chosen here to allow fathoming incredible light with ease in this chapter. As a hint to the overall content of this chapter, an excerpt from the description of “scattering of light” is reproduced below.

Scattering of Light Scattering is a phenomenon wherein light, upon interaction with a particle, microscopic or macroscopic, is redirected in many different directions. Let us take a small digression here and refer to the famed picture called Earthrise, a snap of Earth taken from the lunar orbiter during the Apollo 8 mission, the first crewed voyage to orbit the Moon. This allowed humankind, for the first time ever, to view the sky through the lens of a camera located in the vicinity of a natural celestial body where there is no medium to scatter sunlight. We replicate here the Earthrise (Fig. 2.33), acclaimed as the most influential environmental photograph ever taken, and seek to decode the science this picture holds. The Sun, hiding on the other side of the camera, is obviously the source of light for this picture. The lunar surface at the foreground and the Earth at a distance are seen basically by sunlight reflected off them. Sunlight traverses through the empty space between the Moon and the Earth, and in the absence of any matter to return light back to the camera lens, it appears pitch dark. Even the space all around the Earth, which we perceive as our blue sky, appears no less dark from the moon! The only difference being that we look at the sky through a layer of atmosphere that surrounds the Earth while the moon has no atmosphere. Unmistakably then, our sky would be just as black as it is in this picture if the Earth didn’t have an atmosphere.

Let us dig a little deeper to determine the role the atmosphere plays in this context. Our atmosphere, for instance, is a layer of gases, called air, surrounding the Earth that has been retained by gravity. Air, as we know, is predominantly made up of nitrogen and oxygen molecules. In 1871, Lord Rayleigh (1842–1919), a British physicist, provided a theoretical description of the scattering of light involving particles smaller than the wavelength of the light. According to this theory, now popular as Rayleigh scattering, which paved the way for his securing the 1904 Nobel Prize in Physics, the particle can scatter a part of the incident light uniformly all around it. The intensity of the scattered light (I_s) is inversely proportional to the fourth power of wavelength of the incident light, i.e.:

The white sunlight, as we know, is made up of seven colors of which violet possesses the smallest wavelength and red the largest and the remaining five lie in between. The size of both nitrogen and oxygen molecules, the major constituents of air, is less than even the wavelength of violet light. Clearly, both of these molecules, upon being shone with sunlight, will scatter all seven colors of white light following the description of Rayleigh, meaning that the violet color will be scattered the most and the red the least. However, there is not as much violet or indigo in the sunlight as there is blue. By the time the sunray reaches the Earth’s surface, after traversing the entire atmospheric column, it would have lost a large amount of blue through Rayleigh scattering. Finally, then, as we look away from the Sun into the sky, it is more likely that we would catch the scattered blue light making the sky appear blue. The moon has no atmosphere, so there is nothing to scatter the sunlight, which is why the entire sky in the Earthrise photo appeared absolutely dark.

It is common knowledge that on a clear and cloudless bright day, particularly following a shower, the bluish hue of the sky appears much deeper. The slice of the fascinatingly deep blue sky on a clear and bright sunny morning following a brief but heavy downpour captured in Fig. 2.37 bears testimony to this fact. The rain basically cleans up the air by settling down the dust and other suspended particles from it. The obvious effect of the presence of these impurities, of size far exceeding the light wavelengths, in the air is, therefore, to trim the blue of the sky a shade or two. You may now begin to wonder if Raleigh scattering makes the sky appear blue, what is it that makes the cloud floating in the same sky often appear white? The cloud looks white as the white sunlight gets scattered from the water droplets that the cloud is made up of, preserving all its constituent colors. Similar to dust and impurity particulates, water droplets are also much larger than air molecules and do not seem to follow the Rayleigh scattering formula. In 1908, Gustav Mie (1868–1957), a German physicist, presented a description of the scattering of light by particulates of sizes exceeding its wavelength. Qualitatively speaking, the intensity of scattering, known as Mie scattering which has no restriction on the upper size of the particulates, is invariant of the wavelength of light. This thus readily explains why the presence of particulates in the atmosphere reduces the bluish hue of the sky or for that matter why clouds often appear white.

Graphical Abstract

Fig. 2.33 The rise of Earth as seen from near the Moon. This legendary picture, shot on December 24, 1968, during Apollo 8 lunar mission, is credited to astronaut William Anders (Source: NASA, Wikimedia Commons).

Fig. 2.37 This photograph taken on a bright sunny morning in the San Francisco Bay Area following a shower reveals the captivating blue hue of our sky.

1. 1.

   Plasma is a gas of ions and electrons and possesses a natural frequency of oscillation that increases with the density of the charged particles. It is a common knowledge that a body of plasma allows electromagnetic waves of frequency above its own frequency to pass through and blocks the rest. The density of plasma, in the early Universe of extreme temperature, was much too high and so was also its natural frequency. Sunlight of even high frequency, let alone the lower ones, could, therefore, barely leak through the plasma during this period.

2. 2.

   Islamic golden era is known to have spanned between eighth and fourteenth centuries when Islamic civilization greatly flourished culturally, scientifically, and economically.

3. 3.

   Scientific revolution that primarily took place in Europe during seventeenth and eighteenth centuries led to a proliferation of scientific thoughts transforming our understanding and views on nature.

4. 4.

   A polymath is an individual whose expertise spans over a significant number of disciplines.

5. 5.

   Corpuscular theory of light, proposed by Newton in the early eighteenth century, states that light is composed of tiny particles, called corpuscles, which propagate in straight line with finite velocity and hence possess momentum.

6. 6.

   It is of interest to note here the thought double slit experiment with a single electron that Nobel Laureate Richard Feynman described in now-famous series of lectures at the California Institute of Technology in 1965. Furthermore, the advances in the manufacturing techniques made possible the real-world demonstration of this famed thought experiment several decades later.

7. 7.

   Light travels slowly in glass or water compared to air or vacuum. The electrons present in an optically transparent medium interact with the electric field ubiquitous with light and, in turn, slows it down. The refractive index (r.i.) is an optical property of a medium that basically relates to its power of slowing down light and is expressed as the ratio of the speed of light in vacuum to that in the medium. A medium with higher r.i. is called optically denser with respect to the other, while a medium with lower r.i. is called optically rarer. Light travels faster in water than in glass, so water is optically rarer than glass and conversely glass is optically denser than water.

8. 8.

   David Scott, an American astronaut, who commanded the Apollo 15 lunar mission had placed a retroreflector on the surface of the Moon on July 31, 1971. The unique property of such a reflector to return the reflected beam in the same direction as the incident beam subsequently allowed the measurement of distance between the Earth and Moon to an incredible accuracy of 2 cm, i.e., equivalent to an error less than even 1 part in 20 billion. Incidentally Scott is one of four surviving Moon walkers today.

9. 9.

   At the critical angle of incidence, θ_c, the angle of refraction is 90^°. Snell’s law of refraction therefore yields

   $$ {\mu}_i\sin {\theta}_c={\mu}_r\sin {90}^o\Rightarrow {\theta}_c={\sin}^{-1}\frac{\mu_r}{\mu_i}. $$
10. 10.

    Monochromaticity of a light source is a measure of the purity of the color of light it emits.

11. 11.

    Wave front is the locus of all the points where waves starting simultaneously from the source arrive at a given instant of time. The wave front of a point source will therefore be a sphere, and the rays of light will be normal to the surface of the sphere.

12. 12.

    As the spacing between the two slits is infinitesimally small, the screen can be assumed to be at infinity. Two parallel rays always meet at infinity; therefore, it is implicit that rays “aB₁” and “bB₁” are parallel. Hence,

    $$ {\textrm{bB}}_1-{\textrm{aB}}_1\approx \textrm{bc}=\textrm{dsin}\uptheta $$
13. 13.

    The phenomenon of iridescence occurs when the physical structure of an object causes light waves to interfere and create a rich array of vibrant color. The hue of the color also changes with both the angles of illumination and viewing.

14. 14.

    A dielectric is a nonmetallic substance that either is a poor conductor of electricity or doesn’t conduct at all. Unlike metals, they have no free or loosely bound electrons.

15. 15.

    Heisenberg’s uncertainty principle: There is an inherent limit imposed by the nature on the combined accuracy with which a pair of certain canonically conjugate variables like position and momentum or energy and time can be determined. If effort is expended to measure one variable more accurately at one instant, the other variable becomes intrinsically more unpredictable at that instant. If Δx and Δy are the uncertainties in the measurement of two such variables x and y, then uncertainty principle can be expressed mathematically as Δx Δy ≥ h/4π, where h is the Planck’s constant.

16. 16.

    Far field in a scientific parlance refers to a location so far away from the point of occurrence of a scientific event that its effect can be considered to be independent of distance beyond this point.

17. 17.

    A polarizer is an optical element that at a particular orientation allows light of a specific polarization to pass through while blocking the other polarization. Imparting a 90^° rotation to the polarizer will flip the polarization of the passing light.

18. 18.

    The orthogonality of reflected and refracted rays at the Brewster’s angle of incidence forbids reflection of light withp polarization as it violates the transverse nature of light.

Authors and Affiliations

1. Former Head, Laser and Plasma Technology Division, Bhabha Atomic Research Centre, Mumbai, India

   Dhruba J. Biswas

Reprints and permissions

© 2023 The Author(s), under exclusive license to Springer Nature Switzerland AG

Policies and ethics